Divergence of Water Vapour as a Measure Of The Efficiency of the Rain-Producing Systems in the Sudan

Abstract

Moisture-related fields were retrieved from the archive of ECMWF. Daily moisture budgets were constructed at selected locations over the Sudan during the month of August 1988. The components of the moisture budget were: evaporation, precipitation, changes of the column content of water vapour, the vertical integral of the divergence of water vapour and the sub-grid processes. The efficiency of a rain-producing system was defined as the ratio of the collected precipitation to the total amount of water vapour introduced into that column of air. It was found least at the areas closed to the ITCZ. The efficiency of the large-scale disturbances was found less than average. The contribution of the advocated moisture in the produced precipitation was largest at the northern areas. This quantity decreased westwards and southwards. The advocated moisture was found to build gradually ahead of the area of maximum precipitation and to drop sharply on its rear. The area of maximum convergence of water vapour had two types of movements: A westwards movement and a longitudinal oscillatory movement. The area of maximum convergence shifted to the north prier to the incidence of maximum precipitation and swung southward after the passage of the wave of disturbance. Low level forcings did not produce propagating storms. The study confirmed the idea that low level forcings and large scale forcings work constructively to produce long-lived storms during the peak of the monsoons.